1. Field of the Invention
The present invention relates generally to air conditioners, and particularly to a heat pump air conditioner improved in returning lubricant oil to a compressor at the time of starting a refrigeration cycle. The present invention is suitably applied to a heat pump air conditioner for an electric vehicle.
2. Related Art
Conventionally, a heat pump air conditioner for a vehicle has an outside heat exchanger disposed outside a passenger compartment of the vehicle, an inside heat exchanger disposed inside the passenger compartment, a compressor and a separator. The separator is disposed at a suction side of the compressor for separating refrigerant into gas refrigerant and liquid refrigerant and storing liquid refrigerant therein. In a heating mode, the air conditioner releases heat absorbed by the outside heat exchanger and heat corresponding to an amount of a compression work performed by the compressor into air by the inside heat exchanger, thereby heating the air.
Gas refrigerant in the separator is sucked into the compressor through a gas suction pipe. The gas suction pipe has an oil return hole proximate a bottom of the separator so that oil-dissolved liquid refrigerant is sucked through the oil return hole and introduced into the compressor. As a result, a mixture of gas refrigerant and oil-dissolved liquid refrigerant is introduced into the compressor, and a sufficient amount of oil returned to the compressor.
However, when the vehicle having the heat pump air conditioner has been left at a low temperature such as -10.degree. C. or lower after the air conditioner was stopped, it takes long time for a temperature of the inside heat exchanger to decrease to the same level as a temperature of air outside the passenger compartment (hereinafter referred to as outside air temperature). On the other hand, a temperature of the outside heat exchanger decreases to the same level as the outside air temperature in a short time. Therefore, a saturation pressure of refrigerant flowing through the inside heat exchanger becomes higher than that of refrigerant flowing through the outside heat exchanger.
As a result, while the air conditioner is stopped, refrigerant in the inside heat exchanger flows into the outside heat exchanger through a decompression device such as a capillary tube and stay therein. Therefore, since only little liquid refrigerant and a small amount of gas refrigerant exists on an upstream refrigerant side of the capillary tube, gas refrigerant passes through the capillary tube when the air conditioner is restarted.
However, since a diameter of the capillary tube is set relatively small for a steady-state operation of a refrigerant cycle of the air conditioner, the diameter may become too small for rapidly increasing a flow rate of refrigerant in the refrigeration cycle when starting the air conditioner with the outside air temperature being low. Further, since gas refrigerant passes through the capillary tube less easily than liquid refrigerant, a flow rate of refrigerant in the refrigeration cycle may become too small for an amount of refrigerant discharged from the compressor.
As a result, a suction pressure of the compressor is rapidly decreased as a rotational speed of the compressor is increased at the time of restarting the air conditioner. This causes rapid vaporization or foaming of liquid refrigerant in the separator, and liquid refrigerant in the separator is instantaneously discharged to the suction side of the compressor. Therefore, a surface of liquid refrigerant in the separator may rapidly lowered to become below the oil return hole temporarily, causing that no oil returns from the separator to the compressor temporarily. As a result, the compressor may suffer from oil shortage to have poor lubrication, thereby adversely affecting a durability of the compressor.
The above-described decrease in an amount of oil returning to the compressor occurs even when a sealed amount of the refrigerant in the refrigeration cycle is sufficient in a steady state. Further, when the outside air temperature is lower, or the sealed amount of refrigerant in the cycle is smaller, or the air conditioner is left for a longer time, the amount of oil returning to the compressor is lesser and the oil shortage of the compressor lasts longer.
The amount of oil returning to the compressor may also be decreased in a cooling mode. When the heat pump air conditioner is operated in the cooling mode and is stopped to be left while the outside air temperature is relatively high in summer, the outside heat exchanger is maintained at a high temperature due to the high outside air temperature. On the other hand, the inside heat exchanger is maintained at a relatively low temperature.
As a result, while the air conditioner is stopped, a saturation pressure of refrigerant in the outside heat exchanger becomes higher than that of refrigerant in the inside heat exchanger. Therefore, refrigerant in the outside heat exchanger flows into the inside heat exchanger through a decompression device such as a capillary tube and stay therein. As a result, only little liquid refrigerant and a small amount of gas refrigerant exists on an upstream refrigerant side of the capillary tube. Therefore, when the cooling mode is restarted, a suction pressure of the compressor is rapidly decreased as a rotational speed of the compressor is increased at the time of restarting the compressor similarly to the heating mode. This causes a rapid vaporization or foaming of liquid refrigerant in the separator, and no oil returns from the separator to the compressor temporarily.
Especially, when a deviation rate of a rotational speed of the compressor is increased at the time of starting the cooling mode to improve cooling performance of the air conditioner, the amount of oil returning to the compressor is further decreased.